We continued to use the pUR288 transgenic shuttle vector for the determination of in vivo mutant rates to assess general somatic mutagenesis in mouse B cells. In FY 2002, we completed the backcross of the pUR288 transgene from strain C57BL/6 to strains BALB/c and DBA/2N. The newly generated transgenic/congenic strains will be used in genetic studies to associate the susceptibility to Myc -induced B-cell neoplasia with levels of oxidative mutagenesis in B cells. In FY 2002, we performed three experiments using the pUR288 mutagenesis assay. In the first experiment, we employed mice that co-harbored pUR288 and a low efficiency allele of the major isozyme of glucose-6-phosphate dehydrogenase (G6PD) to further our understanding of the interdependence of general metabolism, oxidative stress control, and somatic mutagenesis. We found that G6PD deficient mice exhibited a significant distortion of redox control, a considerable accumulation of promutagenic etheno DNA adducts, and a substantial elevation of somatic mutation rates in lacZ, the target and reporter gene of mutagenesis in the shuttle vector, pUR288. The mutation pattern in spleen and brain was dominated by illegitimate genetic recombinations. These findings indicated that G6PD is critical for limiting oxidative mutagenesis in the mouse. To evaluate oxidative mutagenesis in early B progenitors, we prepared pre-B cells from fetal livers of BALB/c and DBA/2N mice carrying pUR288. We measured both g-irradiation induced mutagenesis in differentiation-arrested pre-B cells and endogenous mutagenesis in pre-B cells undergoing differentiation into surface IgM expressing immature B lymphocytes. We observed that pre-B cells are extremely susceptible to oxidative mutagenesis, that Bcl-2 is a potent inhibitor of pre-B mutagenesis, and that strain BALB/c is more susceptible to oxidative attack than strain DBA/2. In the third experiment, we studied the mutator phenotype in mouse Burkitt's lymphoma (mBL) induced by the l-MYC trangsgene. We found that the mutant frequency in lacZ was only moderately elevated in mBL; however, the mutational pattern in lacZ exhibited a striking shift from point mutations to recombination mutations. Genomic instability in mBL seems thus characterized by a preponderance of recombination mutations in the context of near-background mutant frequencies. Our finding supports the notion that deregulated Myc may act as a structural modifier of the genome.